Age-associated changes in immune function in humans and animals are quite important with regard not only to the general health of aged persons but also to the general features of the immune system itself. Elderly subjects have been shown to be more susceptible to viral and bacterial infections and are believed to be more susceptible to cancer. A series of clinical studies have revealed that elderly subject, in contrast to their younger counterparts, exhibit poor cellular and humoral immune responses to vaccines even in the presence of standard adjuvants. Currently, many laboratories are focusing their research efforts into developing more effective adjuvants for use in elderly populations. Our laboratory in collaboration with various investigators at the National Cancer Institute has demonstrated that both prolactin and human growth hormone potentiate human and rodent lymphocyte activation and proliferation in response to various antigens and stimuli both in vitro and in vivo. These hormones have also been shown to modulate a variety of leukocyte functions including potentiating neutrophil killing of bacteria and yeast, lymphocyte activation to specific antigens and immune stimuli, modulating cytokine production, enhancing natural killer cell activity, thymic engraftment and regeneration, mast cell and granulocyte degranulation, and augmenting antibody production. Continuing efforts are underway to initiate a prolactin infusion study in young and older subjects to examine their role as immunoadjuvants and pro-engraftment factors. It is our hope to examine the ability of recombinant hormones to strengthen the immune system in healthy individuals in response to vaccines to common pathogens including pneumococcus and influenza.